1. Field of the Invention
The present invention relates to a method of forming a silicon-based thin film, a silicon-based thin film and a photovoltaic element such as solar cell or sensor formed by depositing one set or more of pin junctions.
2. Related Background Art
As a method of forming a silicon thin film showing crystallinity, there have so far been carried out a method such as the cast method based on the growth from a liquid phase, which, however, needs high temperature processing and has a problem to mass production and cost reduction.
As a method of forming a silicon thin film showing crystallinity other than the cast method, there has been known a method of performing a hydrogen plasma processing after the formation of amorphous silicon and repeating this to form a polycrystalline silicon film, disclosed in Japanese Patent Application Laid-Open No. 5-136062.
With a photovoltaic element using a silicon thin film showing crystallinity, it is generally known that the effect of dangling bonds or the like in crystal grain boundaries of silicon, distortions in the vicinity of crystal grain boundaries, imperfectness of crystals themselves and so on hinder the transportability of carriers, thereby badly affecting the photoelectric characteristics as a photovoltaic element.
As countermeasures to reduce the above effects, such contrivances have been required as lowering the film forming rate to improve the crystallizing ratio and the crystallinity or effecting the film formation while repeating the formation of a silicon film and the annealing in a hydrogen atmosphere. These processing has provided a factor for prolonging the film forming time to increase the cost.
Thus, it is an object of the present invention to solve the above mentioned problems and to provide a method of forming a silicon thin film, a silicon thin film and a photovoltaic element that attain excellent photoelectric characteristics and a film forming rate of an industrially practical level.
According to a first aspect of the present invention, there is provided a method of forming a silicon-based thin film comprising effecting high frequency plasma CVD using a source gas comprising a silicon halide and hydrogen, wherein the value of Q defined by Q=Poxc3x97PR/S/d is 50 or more, wherein Po (W) is a supplied power, S (cm2) is the area of a high frequency introducing electrode, d (cm) is a distance between the high frequency introducing electrode and a substrate, and PR (mTorr) is a pressure.
According to a second aspect of the present invention, there is provided a silicon-based thin film formed by high frequency plasma CVD using a source gas comprising a silicon halide and hydrogen, wherein the high frequency plasma CVD is effected under such conditions that the value of Q defined by Q=Poxc3x97PR/S/d is 50 or more, wherein Po (W) is a supplied power, S (cm2) is the area of a high frequency introducing electrode, d (cm) is a distance between the high frequency introducing electrode and a substrate, and PR (mTorr) is a pressure.
According to a third aspect of the present invention, there is provided a photovoltaic element comprising a semiconductor layer comprised of at least one pin junction on a substrate, at least one i-type semiconductor layer being formed by high frequency plasma CVD using a source gas comprising a silicon halide and hydrogen, wherein the element comprises a silicon-based thin film comprising a crystal phase formed under such conditions that the value of Q defined by Q=Poxc3x97PR/S/d is 50 or more, wherein Po (W) is a supplied power, S (cm2) is the area of a high frequency introducing electrode, d (cm) is a distance between the high frequency introducing electrode and a substrate, and PR (mTorr) is a pressure.
It is preferable that the above silicon halide contains at least one element of fluorine and chlorine.
It is preferable that the flow rate of the hydrogen in the above source gas is equal to or greater than the flow rate of the silicon halide.
The above pressure PR is preferably 50 mTorr or more. More preferably it is 100 mTorr or more and most preferably 500 mTorr or more. Its upper limit is preferably 20 Torr.
With the above silicon thin film, the Raman scattering intensity originating in a crystal component is preferably three or more times the Raman scattering intensity originating in an amorphous component.
With the above silicon thin film, the diffraction intensity for (220) by X-ray or electron beam diffraction is preferably 50% or more of the total diffraction intensity. Below 50%, the lowering in crystal grain boundary density becomes remarkable. More preferably it is 60% or more and most preferably 70% or more. Its upper limit is preferably 95%.